Metal finishing process



Patented June 17, 1941 METAL FINISHING PROCESS Gerald G. Romig, Melrose Park, Pa., assignor to American Chemical Paint Company, Ambler, Pa'., a corporation of Delaware No Drawing. Application March -7, 1940, Serial No. 322,757

2 Claims. (Cl. 148-65) This invention relates to coating metal and more particularly it relates to an improvement in methods by which a substantially integral coating is produced by chemical means upon a metal surface which is thereafter to be covered with a coat of paint, varnish, enamel, Japan, or the like, which dries or hardens over the chemical coat and is hereafter referred to, for brevity, as a siccative coat. In particular, the invention relates to the application of siccative coatings to ferrous metal surfaces which have The surfaces should, of course, be dried befor the siccative coating is applied.

In its'preferred form, the chromic acid solution is prepared by dissolving from /2 to 8 pounds had formed or deposited thereon adherent coatings of water-insoluble metallic phosphates.

It is an object of the present invention to improve the adherence of siccative coatings to such metallic phosphate coatings when these have been produced with the aid of certain agents.

A further object of the invention is to prevent the appearance of blisters in the siccative coat when applied over such phosphate coatings produced with the aid ,of certain agents when the siccative coating is exposed to humid atmospheres.

It has been found useful in the production of adherent coatings of zinc phosphate upon a surface of ferrous metal to employ in their production hydrogen peroxide. While the adherence of siccative coatings to the metal is increased by the bonding effect of such phosphate coatings produced in the manner described, it has been found diflicult to avoid blisters between the phosphate coating and the siccative coating where the phosphate coating has been produced i with the aid of hydrogen peroxide, particularly when the finished surface is placed in a highly humid atmosphere. An object of the present invention is to provide an intermediate treatment after the production of phosphate coating by means of a solution containing hydrogen peroxide and before the application of the siccative coating whereby the blistering of this siccative coating is minimized or eliminated.

After the production of the phosphate coating the surface is customarily rinsed before the application of the siccative coating. In accordance with the present'invention, the phosphate coat is previously rinsed in the usual manner and is then treated with a dilute solution of chromic acid. It may be noted that the severity and extent of the blistering ordinarily encountered increases with the degree of contamination of the water used torinse the. phosphate coated sur-' face, particularly with the amount of calcium salts and of chloride ion contained in the water.

of chromic acid per gallons of-water. For ordinary commercial purposes, about 4 pounds of chromic acid per 100 gallons of water is preferred, and while a strength greater than 8 pounds per 100 gallons of water seems unnecessary, it seems to do little if any harm until 8 pounds or over are used. Stronger solutions of chromic acid are likely to injure either the phosphate coating or the subsequently applied siccative coating.

It is preferable to employ the customary rinsing after the production of the phosphate coating and before the use of the chromic acid rinse so as to avoid contamination of the chromic acid rinse. No treatment is necessary after the chromic acid rinse and before the application of the siccative coating. If any subsequent rinsing takes place, it should be with distilled water or at least with water which does not contain sulficient impurities to produce blisters.

The length of treatment in the chromic acid rinse need only be sufficient thoroughly to replace any previous solution or rinse water so that the work may be thoroughly wetted with the chromic acid solution. As the work must generally be dried after the chromic acid rinseand beforethe application of the finish coat, it is preferable to use the chromic acid solution at a temperature approaching the boiling point so that the metal may more readily be dried. However, temperatures as low as room temperature or even lower are perfectly satisfactory provided suitable means are available for thoroughly drying the surface after the chromic acid rinse.

The acid rinse to minimize or eliminate blisters in a siccative coating is particularly efiective when employed with a phosphate coating produced by the action of a phosphate solution containing hydrogen peroxide or some substance which will generate hydrogenperoxide when added to the solution.

Also, while the most important use of the method at the present time is in connection with coating iron or steel, it is also effective with phosphate coatings produced on other metals, for instance, phosphate coatings produced on zinc, magnesium, or their alloys, when they are produced by the aid of hydrogen peroxide or some hydrogen-peroxide-generating substance.

It is to be understood that the rinseof this invention is preferably a substantially pure aqueous solution of chromic acid. However, the presence in such a rinse of moderate quantities of certain other substances, and in particular of such dichromates as those of ammonia and of the alkali metals, may do no harm.

-After the rinse of this invention, as before stated, the metallic surfaces are either. air dried or heat dried before application of the siccative coating.

What I claim is:

1. In the art of finishing ferrous metal, wherein the surface is coated with a water insoluble metallic phosphate by treating it with an acid zinc phosphate solution containing hydrogen peroxide, the method which comprises in combination rinsing said coating with a dilute aqueous solution of chromic acid and then covering the rinsed surface with a siccative coating.

2. The method of claim 1, in which the dilute chromic acid rinse contains from A; to 8 pounds of chromic acid per 100 gallons of water.

GERALD C. ROMIG. 

